1. Field of the Invention
The present invention relates to a multibeam scanning optical apparatus and a multibeam image forming apparatus. It particularly relates to a multibeam scanning optical apparatus and a multibeam image forming apparatus for forming an image on a photosensitive member by irradiating the photosensitive member with a plurality of beams at predetermined intervals in the secondary scanning direction.
2. Description of Related Art
Recently, in the field of an image forming apparatus utilizing electrophotography, there are provided various multibeam image forming apparatuses for forming an electrostatic latent image on a photosensitive member by using a scanning optical unit incorporating an array of light emitting elements capable of emitting a plurality of beams simultaneously. In such an apparatus, the number of revolutions of a deflector (polygon mirror) and the pixel clock frequency are suppressed by simultaneously emitting a plurality of beams at predetermined intervals in the secondary scanning direction, and the system speed is increased to improve the print productivity per unit time.
However, when at least one of the light emitting elements suffers a failure, the use of the multibeam light emitting element array while enjoying its original function is impossible. In such a case, the operation of the apparatus need be stopped, and the scanning optical unit need be replaced.
To solve the above problem, in the image forming apparatus disclosed in Japan Patent Laid Open Publication No. 2001-353897, image forming for a text document is continued even when one of the multibeam light emitting elements suffers a light emission failure, because such a lack of one beam does not exert a significant influence on the outputted image. In the disclosed apparatus, on the other hand, image forming for a photo document is not allowed when such a failure occurs, because it causes a noticeable defect on the outputted image.
In the above image forming apparatus, however, an image is formed while lacking one beam. Therefore, even in the case of a text document, deterioration in the outputted image is inevitable. Further, since the image forming operation cannot be continued in the case of a photo document, the apparatus cannot be used for an urgent matter. Furthermore, the apparatus cannot address the situation in which a plurality of beams are lacked. Moreover, the failure in just one of the light emitting elements eventually necessitates the replacement of the scanning optical unit, which is not economical.